1. Field of the Invention
The present invention relates generally to aqueous dispersions that include a filler. More specifically, the present invention relates to dispersions that are useful in the carpet industry.
2. Background Art
Methods and techniques for carpet construction are known in the art. A variety of types of carpets exist, including tufted and non-tufted carpets. Tufted carpets are composite structures that include yarn (known as a fiber bundle), a primary backing material having a face surface and a back surface, an adhesive backing material and, optionally, a secondary backing material.
Typically, in order to form the face surface of a tufted carpet, yarn is tufted through the primary backing material such that the longer length of each stitch extends through the face surface of the primary backing material. Typically, the primary backing material is made of a woven or non-woven material such as a thermoplastic polymer, most commonly polypropylene.
The face of a tufted carpet is generally manufactured using one of three methods. First, for a loop pile carpet, the yarn loops formed in the tufting process are left intact. Second, for a cut pile carpet, the yarn loops are cut, either during tufting or after, to produce a pile of yarn ends instead of loops. Third, some carpet styles include both loop and cut pile. One variety of this hybrid is referred to as tip-sheared carpet where loops of differing lengths are tufted followed by shearing the carpet at a height so as to produce a mix of uncut, partially cut, and completely cut loops. Alternatively, the tufting machine can be configured so as to cut only some of the loops, thereby leaving a pattern of cut and uncut loops. Whether loop, cut, or a hybrid, the yarn on the backside of the primary backing material typically comprises tight, unextended loops.
The combination of tufted yarn and a primary backing material without the application of an adhesive backing material or a secondary backing material is referred to in the carpet industry as raw tufted carpet or greige goods. Greige goods become finished tufted carpet with the application of an adhesive backing material and an optional secondary backing material to the backside of the primary backing material. Finished tufted carpet can be prepared as broad-loomed carpet in rolls typically 6 or 12 feet (˜2 or ˜4 meters) wide. Alternatively, carpet can be prepared as carpet tiles, typically 18 inches (50 cm) square to 4 ft (1.3 m) square.
The adhesive backing material is typically applied to the backface of the primary backing material to affix the yarn to the primary backing material. In one method, the adhesive backing material is applied by a pan applicator using a roller, a roll over a roller or a bed, or a knife (also known as a doctor blade) over a roller or a bed. When applied properly, the adhesive backing material does not pass through the primary backing material.
The adhesive backing material may be applied as a single coating or layer or as a multiple layer. The extent or tenacity to which the yarn is affixed is referred to as “tuft lock” or tuft bind strength. Carpets with sufficient tuft lock exhibit good wear resistance and, as such, have longer service lives. In order to have good performance characteristics, the adhesive backing material should substantially penetrate the yarn (fiber bundle) exposed on the backside of the primary backing material and should substantially consolidate individual fibers within the yarn. Good penetration of the yarn and consolidation of the fibers leads to good abrasion resistance. Moreover, in addition to good tuft bind strength and abrasion resistance, the adhesive material preferably imparts or allows good flexibility to the carpet in order to facilitate installation of the carpet.
The secondary backing material is typically a lightweight scrim made of woven or non-woven material such as a thermoplastic polymer, most commonly polypropylene. The secondary backing material is optionally applied to the backside of the carpet onto the adhesive backing material, primarily to provide enhanced dimensional stability to the carpet structure as well as to provide more surface area for the application of direct glue-down adhesives.
Alternative backing materials may include foam-cushioning (e.g. foamed polyurethane) and pressure sensitive floor adhesives. Alternative backing materials may also be applied, for example, as webbing with enhanced surface area, to facilitate direct glue-down adhesive installations (e.g., in contract commercial carpeting, automobile carpet and airplane carpet where the need for cushioning is of times minimal). Alternative backing materials can also be optionally applied to enhance barrier protection with respect to moisture, insects, and foodstuffs, as well as to provide or enhance fire suppression, thermal insulation, and sound dampening properties of the carpet.
Known adhesive backing materials include curable latex, urethane or vinyl systems, with latex systems being most common. Conventional latex systems are low viscosity, aqueous compositions that can be applied at high carpet production rates and offer good fiber-to-backing adhesion, tuft bind strength and adequate flexibility. Generally, excess water is driven off and the latex is cured by passing through a drying oven. Styrene butadiene rubbers (SBR) are the most common polymers used for latex adhesive backing materials. Typically, the latex backing system is heavily filled with an inorganic filler such as calcium carbonate or aluminum trihydrate and includes other ingredients such as antioxidants, antimicrobials, flame retardants, smoke suppressants, wetting agents, and froth aids.
Conventional latex adhesive backing systems can have certain drawbacks. As one important drawback, typical latex adhesive backing systems do not provide a moisture barrier. Another possible drawback, particularly with a carpet having polypropylene yarn and polypropylene primary and secondary backing materials, is the dissimilar polymer of latex systems along with the inorganic filler can reduce the recyclability of the carpet. Additionally, the high molecular weights of latex systems can significantly reduce the recyclability.
In view of these drawbacks, some in the carpet industry have begun seeking suitable replacements for conventional latex adhesive backing systems. One alternative is the use of urethane adhesive backing systems. In addition to providing adequate adhesion to consolidate the carpet, urethane backings generally exhibit good flexibility and barrier properties and, when foamed, can eliminate the need for separate underlayment padding (i.e., can constitute a direct glue-down unitary backing system). However, urethane backing systems also have important drawbacks, including their relatively high cost and demanding curing requirements which necessitate application at slow carpet production rates relative to latex systems.
Thermoplastic polyolefins such as ethylene vinyl acetate (EVA) copolymers and low density polyethylene (LDPE) have also been suggested as adhesive backing materials due in part to their low costs, good moisture stability and no-cure requirements. Various methods are available for applying polyolefin backing materials, including powder coating, hot melt application, and extruded film or sheet lamination. However, using polyolefins to replace latex adhesive backings can also present difficulties. For example, U.S. Pat. No. 5,240,530, Table A at Col. 10, indicates that ordinary polyolefin resins possess inadequate adhesion for use in carpet construction. Additionally, relative to latex and other cured systems, ordinary polyolefins have relatively high application viscosities and relatively high thermal requirements. That is, ordinary thermoplastic polyolefins are characterized by relatively high melt viscosities and high recrystallization or solidification temperatures relative to the typical aqueous viscosities and cure temperature requirements characteristic of latex and other cured (thermosetting) systems.
Even ordinary elastomeric polyolefins, i.e. polyolefins having low crystallinities, generally have relatively high viscosities and relatively high recrystallization temperatures. High recrystallization temperatures result in relatively short molten times during processing and, when combined with high melt viscosities, can make it difficult to achieve adequate penetration of the yarn, especially at conventional adhesive backing application rates.
One method for overcoming the viscosity and recrystallization deficiencies of ordinary polyolefins is to formulate the polyolefin resin as a hot melt adhesive. Such a method usually involves formulating low molecular weight polyolefins with waxes, tackifiers, various flow modifiers and/or other elastomeric materials. Ethylene/vinyl acetate (EVA) copolymers, for example, having been used in formulated hot melt adhesive backing compositions and other polyolefins compositions have also been proposed for use in hot melt backing compositions. For example, in U.S. Pat. No. 3,982,051, Taft et al. disclose that a composition comprising an ethylene/vinyl acetate copolymer, atactic polypropylene, and vulcanized rubber is useful as a hot melt carpet backing adhesive.
Unfortunately, hot melt adhesive systems are not generally considered to be complete replacements for conventional latex adhesive backings. Typical hot melt systems of EVA and other copolymers of ethylene and unsaturated comonomers can require considerable effort in formulation and often yield inadequate tuft bind strengths. Furthermore, they require the purchase of new capital as they cannot be run on latex-enabled systems. However, the most significant deficiency of typical hot melt systems is their melt strengths, which are generally too low to permit application by a direct extrusion coating technique. As such, polyolefin hot melt systems are typically applied to primary backings by relatively slow, less efficient techniques, such as by the use of heated doctor blades or rotating melt transfer rollers.
While unformulated high pressure low density polyethylene (LDPE) can be applied by a conventional extrusion coating technique, LDPE resins typically have poor flexibility, which can result in excessive carpet stiffness. On the other hand, those polyolefins that have improved flexibility, such as ultra low density polyethylene (ULDPE) and ethylene/propylene interpolymers, still do not possess sufficient flexibility, have excessively low melt strengths, and/or tend to draw resonate during extrusion coating. To overcome extrusion coating difficulties, ordinary polyolefins with sufficient flexibility can be applied by lamination techniques to insure adequate yarn-to-backing adhesion; however, lamination techniques are typically expensive and can result in reduced production rates relative to direct extrusion coating techniques.
Known examples of flexible polyolefin backing materials are disclosed in U.S. Pat. Nos. 3,390,035; 3,583,936; 3,745,054; and 3,914,489. In general, these disclosures describe hot melt adhesive backing compositions based on an ethylene copolymer, such as ethylene/vinyl acetate (EVA), and waxes. Known techniques for enhancing the penetration of hot melt adhesive backing compositions in the yarn include applying pressure while the greige good is in contact with rotating melt transfer rollers as described, for example, in U.S. Pat. No. 3,551,231.
Another known technique for enhancing the effectiveness of hot melt systems involve using pre-coat systems. For example, U.S. Pat. Nos. 3,684,600; 3,583,936; and 3,745,054, describe the application of low viscosity aqueous pre-coats to the back surface of a primary backing material prior to the application of a hot melt adhesive composition. The hot melt adhesive backing systems disclosed in these patents are derived from multi-component formulations based on functional ethylene polymers such as, ethylene/ethyl acrylate (EEA) and ethylene/vinyl acetate (EVA) copolymers.
Another prior art method for manufacturing carpet is disclosed in PCT Publication No. 98/38376, which discloses an extrusion coating technique that uses a homogeneously branched linear ethylene polymer as a backing material. That application discloses using particle sizes in the 18 to 22 micron range and formulating a particle in water slurry.
Although there are various systems known in the art of carpet backings, there remains a need for a thermoplastic polyolefin carpet backing system, which provides adequate tuft bind strength, good abrasion resistance and good flexibility, to replace cured latex backing systems. A need also remains for an application method that permits high carpet production rates while achieving the desired characteristics of good tuft bind strength, abrasion resistance, barrier properties and flexibility. Finally, there is also a need for a carpet structure having fibers and backing materials that are easily recyclable without the necessity of extensive handing and segregation of carpet component materials.